Been asked to wire a 10.8kw shower at someones house.
They said there is already a cable up in the attic ready to be used. When checking the cable for size i noticed it was 6.0mm so told the customer to either down rate the shower or replace the cable with a 10mm.

Got a phone call later on saying they the customer had spoken to another spark who said the 6.0mm cable would be ok and that it could handle the current up to 29m in length.

Who's right?

Doing a quick calc 10.8kw divided by 240v = 45a

Then there are all the derating factors to be applied thermal insulation, distance etc bringing the current carrying capacity down.

So say the 6.0mm will take 45 amps the next breaker applicable is 50a am i right in saying the cable won't take it?

So for short circuit protection only a 6mm cable covered by a 50A MCB gives you a circuit length maximum of 92 metres.

Rockin! whats the problem

regards

-------------------------
"Take nothing but a picture,leave nothing but footprints!"-------------------------"Oh! The drama of it all."-------------------------"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"-------------------------

For an existing cable and querying a 10.5KW shower a quick glance at OSG/BS7671 gives 47amps clipped direct.. you can get away with the overload protection by applying 433.3.1 assuming the PSCC is catered for by the cable, which for 6mm it likely is.. Its a bit of a fine comfort line mind, but you could argue its OK but not ideal.

I would likely go for a 40amp mcb though.. to give some measure of protection that would be safer than a 50amp MCB and it would be unlikely to trip for the duration showers are on and provide adequate functioning in daily use.. I wouldnt personally put an mcb in place that made the primary circuit attached to it without overload protection... a 40amp mcb I would consider a better option that would retain functionality.

In short I would install the shower and make a statement of reasoning on any certification provided.

433.1 Every circuit shall be designed so that a small overload of long duration is unlikely to occur.

You may say it ain't long, but don't say it ain't likely.

I love to make use of omission, particularly for showers, but aside from taking account of the power quoted being for 240V as opposed to our actual 230V, and therefore lower, I keep that drawn power<= Iz and select the next MCB up for fault protection.

One day sweet lord, I will understand what RnR is saying. Today is no that day and tomorrow looking unlikely.